MAST, the spherical tokamak operated by CCFE, is providing new insights into plasma behaviour thanks to a series of diagnostic upgrades developed in partnership with labs around Europe.

MAST lends itself to excellent plasma diagnosis as the vacuum vessel has many ports with excellent wide views of the plasma. It has recently completed the first campaign of experiments with its enhanced suite of diagnostics and researchers are delighted with the data that have been gained so far.

Beam Emission

A new Beam Emission Spectroscopy (BES) system, the product of a collaboration between CCFE and KFKI-RMKI in Hungary (see Fusion News September 2010), has given physicists the ability to produce previously unseen images of the bubbling turbulence inside the core of the MAST plasma. Irregular fluctuations in the movement of particles from the core to the edge threaten the plasma’s stability and cause unwanted energy losses. Getting a clear picture of this turbulence is therefore essential in understanding and mitigating it. The BES system makes measurements by detecting the light emitted when neutral atoms are injected into the plasma to heat it. The diagnostic’s very high time resolution allows fusion researchers to map the evolution of turbulent structures at small scales. The four images shown here demonstrate this – showing fluctuations in the density of the plasma at five microsecond intervals. “Such images of turbulence in the core of the plasma are only available in a very few fusion experiments worldwide,” explained CCFE’s Dr Anthony Field, who designed the system. “We will now be able to compare our simulations directly with results from MAST for the first time. This will help in modelling the performance – i.e. confinement and stability – of plasmas for next-generation tokamaks, including ITER.”

Neutron measurements

Another recent addition to MAST is more evidence of how international collaborations can pay off. Entering the MAST machine area at Culham, one encounters a four-ton white polythene box attached to the tokamak. Although reminiscent of an ancient monolith, the box is in fact a highly advanced ‘camera’ containing a set of detectors to view neutrons escaping from the plasma. Neutrons are mainly produced by fusion reactions involving fast ions generated by the neutral beam system. Measuring the neutrons gives valuable information about the fast ions that formed them. These fast ions can, for instance, drive the formation of instabilities and it is important to determine their speed and other parameters. The neutron camera was a joint effort with Uppsala University in Sweden, who developed the detectors in the camera and the data acquisition system and helped design the camera with CCFE. “The project has given Uppsala a very good opportunity to participate in an international experiment like MAST,” said Dr Marco Cecconello of Uppsala University. “The collaboration with Uppsala is an example of how we can share expertise and expenses around Europe in order to get the best results for the project,” added CCFE’s Dr Mikhail Turnyanskiy.

With a major upgrade to MAST due to begin in 2013, there will be even greater scope for involvement from research colleagues around Europe.

Nick Holloway, CCFE